Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Friday, November 25, 2011

How late is too late?

In pre-industrial Finland, the average woman was caught in a squeeze play. If she married too young, she and her partner might not have enough resources to start a family. If she married too old, she risked genetic extinction. None of her children might survive to adulthood and have children of their own. Source

In a previous post, I discussed how Western European societies used to postpone the age of first reproduction to the mid-20s. When this cultural pattern began is unsure, but it certainly preceded the Black Death of the 14th century and may have existed as early as the 9th century. It seems to have resulted from a combination of land scarcity and the rule of primogeniture, i.e., farms were kept intact and handed over to the eldest son when the parents died or retired. In such a situation, young couples had to wait until they had a farm of their own—and the means to start a family.

How late can a woman postpone having her first child and still be sure of perpetuating her lineage? In her mid 30s? That answer might be true today. In pre-modern societies, however, women had to start earlier. A recent study by Liu and Lummaa (2011) puts it no later than 30. Beyond 30, a woman would be faced with declines in both offspring quantity and offspring quality. Her risk of genetic extinction was proportionately higher.

In a study of rural Finnish parish records from the 18th and 19th centuries, Liu and Lummaa (2011) found that the women were 26 years old on average when they first gave birth. They had thus foregone the first decade of their reproductive life, something that would be unheard of in most traditional human societies. Yet they nonetheless managed to have 6.54 children on average.

Among these women, offspring quantity decreased with increasing age of first reproduction (AFR). This was partly because the time window for reproduction was narrower and partly because more of the reproduction was taking place during years of reduced fertility. Offspring quality held constant until the age of 30 and then too decreased.

The study defined “offspring quality” as the probability that one’s children will survive to adulthood and have children of their own. On average, 60% of the offspring survived to 15 years of age and 47% had children of their own. AFR did not affect offspring quality among mothers under 30. Over 30, higher AFR was associated with a lower probability of the children surviving to adulthood and a lower probability that the surviving children would have families of their own.Why did fewer of these children survive to adulthood? The data provide no direct answers. One reason may be that older mothers tend to have children with lower birth weights, which in turn may lead to early death. Older mothers are also at risk of having children with birth defects.

And why did fewer of the surviving children have children of their own? Again, the data provide no direct answers. It may be that many of these children were physically or behaviorally compromised and thus less able to attract potential mates.

What does all of this mean for us today? In the short term, it means that a large part of the current population is headed towards genetic extinction. In the long term, there will be selection for increased fertility at older ages:

In today´s society however, women do not start childbearing until an older age as marriage is often delayed, and casual or short-term relationships and divorce are more common. As a result, the natural selection maintaining young-age fertility might weaken and the relative strength of natural selection on old-age fertility could increase, something that could potentially lead to improvements in old-age fertility over many generations.

Duncan Gillespie from the University of Sheffield´s Department of Animal and Plant Sciences, said: "In today´s society, family-building appears to be increasingly postponed to older ages, when relatively few women in our evolutionary past would have had the opportunity to reproduce. As a result, this could lead to future evolutionary improvements in old-age female fertility.” (Davis, 2011)

20 comments:

Ben10
said...

"In the long term, there will be selection for increased fertility at older ages"

Hmmm...teen pregnancy is still very high. I was teaching in high school with a majority of hispanic students. There were 6 hipanic girls, all younger than 18, all of them were pregnant, some 'very' pregnant. And an indiscretion from a student boy revealed that half of them were pregnant for the second time.

"Girls in the United States are entering puberty at earlier ages than they have in the past, a new study reports.

More than 10 percent of white 7-year-old girls in the study, which was conducted in the mid-2000s, had reached a stage of breast development marking the start of puberty, compared to just 5 percent in a similar study conducted in the early 1990s.

Black and Hispanic girls continue to mature faster than white girls, on average. Nearly one-quarter of black girls and 15 percent of Hispanic girls had entered puberty by age 7, according to the new study, which appears in the journal Pediatrics."

Maybe the problem with regard to later born children having fewer children of their own is due in part to birth order? I know some studies have found that the more older brothers a man has the more likely he is to be homosexual regardless of the mother's age at childbirth. If this holds true in a more general sense, i.e. If men with more older brothers are progressively less masculine, that might have an effect.

Maybe the problem with regard to later born children having fewer children of their own is due in part to birth order? I know some studies have found that the more older brothers a man has the more likely he is to be homosexual regardless of the mother's age at childbirth. If this holds true in a more general sense, i.e. If men with more older brothers are progressively less masculine, that might have an effect.

Gillespie would be right except he's ignoring the openness of the late childbearing (or no childbearing) communities to superior reproducers. It would require a closed system to produce the effect he predicts, I think.

As far as women delaying marriage and childbirth, I'm noticing several associated trends, which could be interesting topics to discuss sometime in future posts:

1) As women can now provide for themselves, they're no longer seeking men's resources. It used to be that women were valued for their youth (reproductive fitness) and men for their ability to provide resources (wealth). David Buss and other evo-psych researchers have written volumes on this. That is *no longer* the case; men are no longer valued for their resources to the degree they were before. Women now can obtain resources themselves, thanks to having a good education and high-paying jobs.

In this scenario, women selecting men would be looking at other, new factors, such as looks, which are not related to resources.

- Women delaying marriage/family does not imply women delaying sex. In this situation, the "80%-20%" rule kicks in: the top 20% of males are getting access to 80% of the women, with the majority of men ('beta' males) shut out of the market. Why? In a formally monogamous society such as our own, all women can't marry the same top-tier men, so they will have to compromise and consider the whole array of men if forced to think about marriage. But as long as that's in the distant future, they will gravitate to the top-tier 20% for casual sex.

As women can now provide for themselves, they're no longer seeking men's resources. It used to be that women were valued for their youth (reproductive fitness) and men for their ability to provide resources (wealth). David Buss and other evo-psych researchers have written volumes on this. That is *no longer* the case; men are no longer valued for their resources to the degree they were before. Women now can obtain resources themselves, thanks to having a good education and high-paying jobs.

You've brought up two key points that I should have mentioned in my post:

1. We live in an open demographic system. Although there is selection for late childbearing in our dominant culture, this selection is being cancelled out by the inflow of people from early childbearing cultures.

2. Selection for late childbearing co-exists with selection for early sexual maturity and "accidental" childbearing out of wedlock. So we may be moving toward a situation where most childbearing is "accidental."

"'Men are now expressing some traditionally female attitudes, while women are adopting some of those long attributed to men'... "

"Across every age group, women [these days] want more independence than men in their relationships: 77% of women say having their personal space is "very important," vs. 58% for men; 78% of women say the same about having their own interests and hobbies (vs. 64% for men). And 35% of women (vs. 23% of men) say regular nights out with the guys/girls are important."

"Women are [currently] much more interested in their independence than men are."

"Women's enthusiasm for marriage has faded in the light of their growing economic independence. 'For them more than men, marriage has to be good or it's not worth trading their newfound independence for.'"

In summary, we are entering a brave new world. I can attest that even as recently as 5-6 years ago the situation was a lot different.

About the trend for earlier sexual maturation in white european girls, I suggest that this is also the result, at least in part, of their exposure with non-white sexually mature boys in school.While most white boys in their early teen (10-13) will prefer to play starwars than flirting with girls, hispanic boys are much more mature and often harass white girls classmates. Could this display of pheromones and early sexual behavior stimulate white girls sexual maturation? I think so. Of course politically correctness would pretend there's no problem there and refuse furhter investigation while the economic situation will ensure that more and more white girls will go to public school to meet this forced exposure.Once, I have seen in a high school a pretty white girl (~16-18old) parading in front of the Main Office with her black baby in her arms, promoting interracial teen pregnancy i guess. With a white baby she should have been advised for more modesty and discretion but in this case, with an assistant principal, teachers, secretary and even a police officer in the office, all of them pretended to look elsewhere and saw nothing...disgusting. I was one step to notify the principal of that when, considering my miserable and insecure economic situation, i realised that complaining to the Affirmative Action-appointed african american principal would probably do me no good. I assumed he was promoted through AA because his 7-foot tall physical, before his fat salary made him fatter, suggested an early football carrier rather than excellence in maths or sciences. And that i know because i never had a 7 foot tall african american in math that was not sleeping, chatting or checking on white girls.

I do think that some of it has to do with late mernarche. It looks that at least in the early XX century women where having theirs menarches with 15 to 18 years while now it's more common to a few years earlier.

Girls are reaching sexual maturity at an earlier age, as seen in the falling age of menarche. This is probably due to increasing caloric intake. Menarche doesn't occur until a girl accumulates a certain critical mass of body fat. If she has that fat at an earlier age, the process of sexual maturation will be accelerated.

But I can't help wondering whether that's the whole story. It has also been shown that menarche comes earlier among girls raised by stepfathers than among girls raised by biological fathers. It used to be thought that the stepfather's presence was exercising some kind of direct effect. Jane Mendle, however, has shown that girls raised by stepfathers tend to have mothers with lower ages of menarche.

In other words, a lower age of menarche is part of a broader reproductive strategy that includes low paternal investment, high maternal investment, and weaker pair bonds.

On a psychological level, this strategy means that sons will show hypermasculine behavior, such as aggressive acting out, boasting, and risk-taking. Daughters will judge potential mates by current appearance and status in the male hierarchy rather than by steadfastness and ability to support a family.

Are we seeing a shift towards this kind of reproductive strategy in the general population?

"...It has also been shown that menarche comes earlier among girls raised by stepfathers than among girls raised by biological fathers..."

What about a direct biological effect? Biological fathers produce inhibitory developmental signals for sexualization under the form of inhibitory behavior (competition mother-daughter in favor of the mom) or inhibitory hormonal (uncompatible related pheromones), while a step father produces the oposite signals, i.e, competition mother-daughter which quite often results in the daughter being abused and stimulating compatible body pheromones.

there are some indications that autism is perhaps connected to having older parents.

so, i've been wondering if there might be higher numbers of people on the autistic spectrum in some european populations. after all, it's not like the peoples behind the hajnal line are known for their social skills. (~_^)

Interesting recent article with lots of facts on the impact of the contraceptive pill on Britain:

http://www.bbc.co.uk/news/uk-15984258

"In 1961, women's lives were very different.

Often married at an early age, most women were expected to stay at home and raise an expanding family while men went out to work.

Nowadays, women can choose to have children, further education and a career on their own terms.

The pill was instrumental in changing that.

"I don't think people thought it would be as revolutionary as it was," says Dame Valerie Beral, a professor of epidemiology at Oxford University, who has spent the last 40 years researching the pill."

"When introduced on the NHS, the pill was prescribed mainly to older women who already had children and did not want any more. The government at the time did not want to be seen to be encouraging promiscuity or "free love".

Although there were not any restrictions on its use, the take-up of GPs prescribing it was slow.

That all changed in 1974 when family planning clinics were allowed to prescribe single women with the pill - a controversial decision at the time."

"Now, according to the latest prescribing review, two million women take it in England and Wales. It is estimated that 70% of all women in Britain have used the pill at some stage in their lives."

"In Britain in the early 1960s, fewer than one in 100 adults under 50 were estimated to have cohabited, whereas nowadays about one in six do, according to a report by the CPC."

- Babies born to women aged under 25 in England and Wales fell from 47% (369,600 live births) in 1971 to 25% (180,700 live births) in 2008

- Historically women's child rearing years were in their early 20s but they are later - in Scotland, the number of births in 2008 among those aged 35 and over exceeded births to women in their early 20s

- In 2008, there were 320,800 live births outside of marriage in England and Wales (45% of all births), in 1971, this figure was less than 10% (67,500)

- Of the women born in 1941, 94% were married by the age of 35, whereas only 60% of women born in 1973 had married by that age

Welcome to my blog! For the most part, this page will be an extension of my website, with comments relating to my research. But it will also branch out into more general discussions of human evolution.